1. Field of the Invention
The present invention relates to the field of a semiconductor device, and more particularly, to a semiconductor memory module.
2. Description of the Related Art
A semiconductor module includes a plurality of semiconductor memory devices that are mounted to a printed circuit substrate as a single unit. Depending on a user""s needs, various memory devices such as dynamic random access memory (DRAM), non-volatile static RAM (SRAM) or video RAM (VRAM) can be used. Among them, a semiconductor memory module using a dynamic random access memory (DRAM) is most widely used.
FIGS. 1 through 5 are diagrams for illustrating a conventional semiconductor memory module with a bolt/nut 16, and its problems.
FIG. 1 is a plan view of a conventional semiconductor memory module. FIG. 1 shows a semiconductor memory module including a module circuit unit 10, a linking unit 12 and a pin connector unit 14. The module circuit unit 10 includes a plurality of semiconductor memory devices mounted on a module board 18 to form a single circuit. If the semiconductor memory module includes an upper heat sink and a lower heat sink 20 and 22, a bolt/nut 16 is used to couple (fix) the two heat sinks together. The pin connector unit 14 is where a pin connector is formed so as to connect the semiconductor memory module to another printed circuit board.
FIG. 2 is a cross sectional view of the semiconductor memory module, taken along the line II-IIxe2x80x2 of FIG. 1. Referring to FIG. 2, in the semiconductor memory module, upper and lower semiconductor devices 26 and 28 such as a ball grid array (BGA) package or a chip scale package (CSP), are mounted on either of two opposing surfaces (top or bottom) of the module board 18, which is a type of printed circuit substrate, using solder balls as outer connection terminals. The upper and lower semiconductor devices 26 and 28 are connected to the upper and lower heat sinks 20 and 22, respectively, via a thermal interface material (TIM) layer 30.
The upper and lower heat sinks 20 and 22 are configured to enclose the module board 18 on which the semiconductor devices 26 and 28 are mounted. As discussed above, the upper and lower heat sinks 20 and 22 are coupled together with a bolt/nut 16.
FIG. 3 is an enlarged view of the portion of the cross section marked as 3 in FIG. 2. FIG. 3 shows the thermal interface material (TIM) layer 30 disposed between the semiconductor devices 26 and 28 and the heat sinks 20 and 22 to facilitate heat transfer between the semiconductor devices 26 and 28 and the heat sinks 20 and 22, respectively.
FIG. 4 is an enlarged view of the portion of the cross section marked as 4 in FIG. 2. Referring to FIG. 4, the upper heat sink 20 and the lower heat sink 22 are coupled together with bolts/nuts. Consequently, the semiconductor devices 26 and 28 are mounted on either of two opposing surfaces of the module board 18 and the upper and lower heat sinks 20 and 22 are fixed to each other using the bolt/nut 16 that are made of metal.
FIG. 5 is a cross sectional view showing a transformed shape of a conventional semiconductor memory module after a temperature cycling test.
In general, semiconductor devices are mounted in integrated electronic equipment systems consisting of hundreds to tens of thousands of components, such as a rocket, a spacecraft, an airplane, and a computer. Even if only one of the semiconductor devices mounted in an integrated electronic equipment system operates incorrectly, the integrated electronic equipment system may malfunction and need to be scrapped.
Therefore, the reliability of each semiconductor device must be ensured before it is used. To solve this problem, semiconductor device manufacturers must perform a reliability test on each of the semiconductor devices before providing the finished goods to customers.
There are many kinds of reliability tests, such as a temperature cycling test, which is usually performed to test the reliability of semiconductor devices as a function of temperature. The temperature cycling test preliminarily tests whether physical and electrical defects occur in semiconductor devices. For this, semiconductor devices are put into a chamber and are then subjected to a temperature cycle of between xe2x88x9265xc2x0 C. and 150xc2x0 C.
However, with a conventional semiconductor memory module, the bonding strength of the solder balls 32 which connect the semiconductor devices 26 and 28 to the module board 18, (the solder joint reliability (SJR)), deteriorates after the temperature cycling test. This is because components of the memory module have different coefficients of thermal expansion (CTE) relative to each other. Specifically, the CTE of silicon, the module board 18 and aluminum forming the upper and lower heat sinks are 2.6 ppm/xc2x0 C., 18 ppm/xc2x0 C., and 24 ppm/xc2x0 C., respectively.
Accordingly, when the semiconductor memory module is subjected to the temperature cycling test, the module board 18, the semiconductor devices 26 and 28, and the upper and lower heat sinks 20 and 22 are alternately contracted and expanded. Their sizes change by different amounts depending on their CTE. For example, the TCE of the module board is higher than that of the silicon forming the semiconductor devices 26 and 28. Thus, the degree (magnitude) of contraction or expansion of the module board is greater than that of silicon.
However, as shown in FIG. 5, solder balls 32xe2x80x2 are fixed between the semiconductor devices 26 and 28 and the upper and lower heat sinks 20 and 22 such that the semiconductor devices and the heat sinks cannot be moved responding to such contraction or expansion. In addition, bolt/nut 16 fixes each of the heat sinks 20 and 22 and the module board 18 together. For these reasons, the solder balls 32xe2x80x2 cannot withstand the force of the contraction and expansion and can be easily disfigured.
Consequently, the solder balls lose their bonding strength between the semiconductor devices 26 and 28 and the module board 18, thereby causing fatal defects in the operation of the semiconductor memory module and severely deteriorating the reliability of the semiconductor memory module.
Accordingly, the present invention provides a memory module that reduces the deterioration of solder joint reliability, which is caused by the differences in the thermal expansion coefficients of components of the memory module.
According to one embodiment, the present invention includes a module board where a plurality of semiconductor devices are stacked, an upper heat sink in contact with the top surface of the semiconductor devices and encloses the top surface of the module board, a lower heat sink in contact with the top surface of the semiconductor devices and encloses the bottom surface of the module board, and a linking means for fixing the upper and lower heat sinks together. The linking means absorbs different degrees (variations) of contraction and expansion of the upper and lower heat sinks and the module board that occurs due to the difference in the thermal expansion coefficients between them.
The linking means can consist of a first coupling body and a second coupling body configured to be coupled (interlocked) together. When the two coupling bodies are coupled together, a gap having a predetermined width is formed at lateral sides of the linking means and extends in a direction parallel to the direction in which the two coupling bodies are coupled.
According to the present invention, the linking means can absorb different degrees of contraction and expansion of the components forming the semiconductor memory module, thereby improving the reliability of the semiconductor memory module including the solder joint reliability (SJR).